How to add equally wide border to a plane?
For this specific case, this is my solution using Hooks &amp; Vertex parenting.
I am not sure whether there is an easier solution but this should work.
I made 4 empties, each is parented to corresponding corner-vertex.
Then for the border object, I made vertex groups also for each corner and used Hook modifiers to link them to empties.





Here is a version with copy rotation constraints to fix the rotation issues: